There are several multiple access modulation techniques for facilitating communications in which a large number of mobile user are present. These techniques include time division multiple access (TDMA), code division multiple access (CDMA) and frequency division multiple access (FDMA).
In TDMA radio telecommunication systems, the communication on the radio path is time-divisional and occurs in successive TDMA frames each of which consists of several time slots. A short information packet is transmitted in each time slot in the form of a radio-frequency burst that has a limited duration and that consists of a number of modulated bits. The time slots are primarily used for conveying control and traffic channels. The traffic channels are used for transferring speech and data. The control channels are for signalling between a base station and mobile stations. An example of a TDMA radio system is the Pan-European digital mobile system GSM (Global System for Mobile Communications). CDMA is a modulation and multiple access scheme based on spread spectrum communication. Unlike FDMA or TDMA, in CDMA a large number of CDMA signals (users) simultaneously share the same wide band radio channel, typically 1.25 MHz. Pseudorandom noise (PN) binary codes, so called spreading codes, are used to distinguish between different CDMA signals, i.e traffic channels on said wide band radio channel. A separate spreading code is used over each connection between a base station and a subscriber terminal. In other words, the narrow-band data signal of the user is conventionally multiplied by the dedicated spreading code and thereby spread in bandwidth to the relatively wide band radio channel. The signals of the users can be distinguished from one another in the receivers on the basis of the unique spreading code of each connection, by using a correlator which accepts only a signal energy from the selected spreading code and despreads its spectrum into a narrow-band signal. The other users' signals, whose spreading codes do not match, are not despread in bandwidth and as a result, contribute only to the noise and represent a self-interference generated by the system. The spreading codes of the system are preferably selected in such a way that the codes used in each system cell are mutually orthogonal, i.e. they do not correlate with each other. Thus, in the CDMA systems, the spreading code unique to each user or user' signal provides a traffic channel in a similar sense as a time slot in the TDMA systems. CDMA is described in more detail in the document: "An overview of the application of code division multiple access (CDMA) to digital cellular systems and personal cellular networks", Qualcomm Incorporated, 1992, USA, (Document Number EX60-10010).
In traditional TDMA and CDMA mobile communications systems, the maximum data rate at the radio interface is relatively low.
In conventional mobile systems, one traffic channel time slot is allocated for communication to each mobile station for the transmission of data or speech. For example the GSM system may therefore comprise as many as eight parallel connections to different mobile stations on a radio frequency carrier. The maximum data transfer rate on one traffic channel is limited to a relatively slow level, e.g. in the GSM system 9.6 Kbps or 12 Kbps, according to the available bandwidth and the channel coding and error correction employed in the transmission. In the GSM system, a so-called half-rate (max. 4.8 Kbps) traffic channel can also be selected for low speech coding rates. The half-rate traffic channel is established when a mobile station operates in an assigned time slot only in every other frame, i.e. at half the rate. Another mobile station operates in the same assigned time slot of every other frame. The system capacity, measured in the number of mobile subscribers, can thus be doubled, i.e. as many as 16 mobile stations can operate on the carrier frequency simultaneously.
In recent years, the need for high-speed data services in mobile networks has increased considerably. For example transmission rates of at least 64 Kbps would be required for the ISDN (Integrated Services Digital Network) circuit-switched digital data services. The data services of the public switched telephone network (PSTN), for example a modem and G3-type telefax terminals, require higher transmission rates such as 14.4 Kbps. One of the increasing areas of mobile data transmission that requires transmission rates exceeding 9.6 Kbps is mobile video services. Examples of such services include security surveillance by means of cameras, and video databases. The minimum data rate in video transmission may be for example 16 or 32 Kbps.
The transmission rates of the present mobile networks are not sufficient for meeting these new requirements, however.
An arrangement, disclosed in Applicant's co-pending international patent application WO95/31878, (unpublished on the filing date of the present application), relates to allocating two or more parallel traffic channels (subchannels) on the radio path for one high-speed data connection. The high-speed data signal is divided in the transmitter into these parallel subchannels for the transmission over the radio path, to be restored in the receiver. This approach enables the supply of data transmission services with as high as eight-fold transmission rate compared to the conventional rate, depending on the number of the traffic channels allocated. For example in the GSM system, the total user data rate of 19.2 Kbps is obtained by two parallel 9.6 Kbps subchannels, each channel being rate-adapted in the same manner as in the existing transparent 9.6 Kbps bearer services of the GSM system.
A problem relating to the use of parallel traffic channels is the data rates which cannot be rate-adapted with the existing methods of the mobile communications system even though these data rates can be evenly distributed between the available parallel subchannels.
In the GSM system, for example, the user data rate of 14.4 Kbps (according to e.g. ITU-T Recommendation V.32bis) requires two transparent GSM traffic channels the data rate in each of which should be 7.2 Kbps (2.times.7.2 Kbps=14.4 Kbps), but there is no rate adaptation in the GSM system for the subchannel data rate of 7.2 Kbps.
Correspondingly, for example the user data rate of 40 Kbps (ITU-T Recommendation V.120) requires five transparent GSM traffic channels in each of which the data rate should be 8 Kbps (40 Kbps: 5), but there is again no rate adaptation in the GSM system for such a subchannel data rate.
Another problem is the data rates that cannot be evenly divided into a required number of traffic channels. In the GSM system, for example, the user data rate of 56 Kbps (ITU-T Recommendation V.110) requires at least six transparent GSM traffic channels, but it cannot be divided into these six parallel subchannels in such a way that the (V.110) frames of each subchannel carry the same number of data bits (56 Kbps: 6=9333.333 bps).